When an electronic part is mounted, a joining method using solder (a solder paste) is widely used.
By the way, in the Sn—Pb solder previously widely used, a bonding method with temperature hierarchy is widely applied, in which soldering is performed at a temperature of 330° C. to 350° C. by using Pb-rich Pb-5Sn (melting point: 314° C. to 310° C.) or Pb-10Sn (melting point: 302° C. to 275° C.) as high temperature solder, for example, and then soldering is performed at a temperature equal to or lower than the melting point of the above-mentioned high temperature solder by using Sn-37Pb eutectic (melting point: 183° C.) of low temperature solder, for example, and thereby, connection by soldering is performed without melting the high temperature solder used in the previous soldering.
Such a bonding method with temperature hierarchy is applied to a semiconductor device of a type in which a chip is die-bonded or a semiconductor device which is subjected to flip-chip bonding, and it is an important technique used for the case where connection by soldering is performed in a semiconductor device, and then the semiconductor device itself is connected to a substrate by soldering.
As a solder paste used for this use, for example, a solder paste including a mixture of (a) second metal (or alloy) balls composed of a second metal such as Cu, Al, Au, or Ag or a high melting point alloy containing the second metal, and (b) first metal balls composed of Sn or In is proposed (refer to Patent Document 1).
Further, in Patent Document 1, a joining method using a solder paste and a method for producing an electronic equipment are disclosed.
When soldering is performed by using the solder paste described in Patent Document 1, as schematically shown in FIG. 2(a), the solder paste including low melting point metal (e.g., Sn) balls 51, high melting point metal (e.g., Cu) balls 52 and a flux 53 is heated to react, and after soldering, as shown in FIG. 2(b), a plurality of the high melting point metal balls 52 are connected to one another with an intermetallic compound 54 formed between a low melting point metal originating from the low melting point metal ball and a high melting point metal originating from the high melting point metal ball interposed therebetween, and by this connecting body, joining objects are connected (soldered) to each other.
In the case of the joining method using the solder paste described in Patent Document 1, the intermetallic compound between the high melting point metal (e.g., Cu) and the low melting point metal (e.g., Sn) is produced by heating the solder paste in the soldering step, and in the combination of Cu (high melting point metal) and Sn (low melting point metal), a large amount of Sn being a low melting point metal remains because the diffusion rates of these metals are slow. When Sn remains, the joint strength under elevated temperatures is significantly deteriorated, and there may be cases where the paste cannot be used depending on the type of the product to be joined. Further, there is a possibility that Sn remaining after the step of soldering may be melted and flowed out in the subsequent another soldering step, and there is a problem that this soldering is low in reliability as high temperature solder which is used for a bonding method with temperature hierarchy.
That is, for example, in a manufacturing process of a semiconductor device, when a semiconductor device is manufactured after undergoing a step of soldering, and then the semiconductor device is to be mounted on a substrate by a reflow soldering method, there is a possibility that Sn remaining after the step of soldering in the manufacturing process of a semiconductor device is melted and flowed out in the step of reflow soldering.
Further, it is necessary to heat the solder paste at a high temperature for a long time in the soldering step in order to convert the low melting point metal entirely to the intermetallic compound so that Sn may not remain, but this heating is practically impossible in consideration of the balance with productivity.
Moreover, when the solder paste described in Patent Document 1 is used, as shown in FIG. 3, an intermetallic compound 64 such as Cu3Sn or Cu6Sn5 is formed in a laminar form at an interface between joining objects 61, 62 after reflowing and a joining material (solder) 63. When such a laminar intermetallic compound 64 is formed, stress concentrates at the interface, and therefore the joint strength of the interface is deteriorated due to production of cracks or the like.    Patent Document 1: Japanese Patent Laid-open Publication No. 2002-254194